The invention described herein was made in the course of, or under, Contract No. W-7405-ENG-48 with the United States Energy Research and Development Administration.
This invention relates to laser fusion targets, particularly to foam-encapsulated laser fushion targets, and more particularly to a method for foam encapsulating laser targets.
Laser-induced fusion has recently joined magnetic-confinement fusion as a prime prospect for generating controlled thermonuclear power. During the past few years much effort has been directed toward the development of fusion fuel targets or capsules and laser-initiated systems for causing implosion and thermonuclear burn of such capsules. Compression of fuel capsules or targets by laser to initiate thermonuclear fusion is described in detail, for example, in an article published in Nature, Vol. 239, No. 5368, pp. 139-142, Sept. 5, 1972 entitled "Laser Compression of Matter to Super-High Densities: Thermonuclear (CTR) Applications" by J. H. Nuckolls et al, and an article published in Physics Today, Aug. 1973 entitled "Laser-Induced Thermonuclear Fusion" by J. H. Nuckolls et al.
Recent efforts have also been directed to the fabrication of laser-fusion targets or fuel capsules compatible with laser energy and cost requirements for commercial power production, as described briefly in the above-cited Nature article and in greater detail in an article entitled "Spherical Hydrogen Targets for Laser-Produced Fusion" by I. Lewkowicz, J. Phys. D: Appl. Phys., Vol. 7, 1974. In addition, the fabrication of laser-fusion targets is discussed in an article entitled "Hydrogen Pellet Generation for Fusion Research" by C. D. Hendricks, "Production of Solid Pellets for Laser Initiated Target Plasma In BBII" by A. L. Hunt et al, and "Hollow Hydrogen Shells for Laser-Fusion Targets" by C. D. Hendricks et al, published in the Bulletin of the American Physical Society, Series II, Vol. 19, Oct., 1974, pages 915, 963 and 927, respectively.
The success of any laser-fusion system for the production of useful power depends critically on the production of suitable targets or fuel capsules which must satisfy a number of requirements in addition to low fabrication cost. Of critical importance to achieving sufficiently high energy gain are the effects of plasma instabilities, fluid instabilities, and the symmetry of the implosion. Thus, the target must have characteristics such as material composition, structure, and surface finish which are compatible with the laser pulse length, energy, peak and average power, and pulse shape.
The initial phase of a laser-driven implosion is the formation of an atmosphere surrounding the target, and thus efforts have been directed to various target configurations which are capable of producing this atmosphere, as well as other means such as gas injection, ablating of the explosive-chamber wall surfaces, etc., which produce a desired atmosphere about the target or fuel capsule. Copending U.S. Pat. applications Ser. No. 609,841 filed Sept. 2, 1975 entitled "Laser-Fusion Targets for Reactors"; and Ser. No. 609,639 filed Sept. 2, 1975 entitled "Foam Encapsulated Targets", each assigned to the assignee of this application, exemplifies laser target configurations capable of producing the desired atmosphere.
Formation of low density foams and encapsulation of microspheres and fillers therein are known in the art, as exemplified by U.S. Pat. No. 3,849,350 issued Nov. 19, 1974.
Conventionally known foams, such as polystyrene, polyurethane, polyethylene, etc., can be obtained in low densities between 0.01 and 0.1.times. 10.sup.3 kg/m.sup.3. However, the cell size is generally too large, 25 to 200 .mu.m, to meet the requirements for foam-encapsulated laser targets which require a foam having both low density and cell size of less than 10 .mu.m, preferably less than 2 .mu.m. Thus, a need has existed in the prior art for a low density, very small cell size foam and method of encapsulating fusion fuel therein.
Copending U.S. Pat. application Ser. No. 609,641, filed Sept. 2, 1975, assigned to the assignee of this application, and now U.S. Pat. No. 4,012,265, describes and claims a foam which fills the above need and meets the requirements for target encapsulating foam. Also, copending U.S. Pat. application Ser. No. 609,642, filed Sept. 2, 1975, assigned to the assignee of this application, describes and claims an extrusion method for fabricating foam-encapsulated laser targets utilizing the foam of said application Ser. No. 609,641 and now U.S. Pat. No. 4,012,265. The targets produced by the extrusion process which produces a fine stand of foam containing fusion fuel-filled shells or spheres distributed along its length have been satisfactory, with laser illumination thereof varifying the effectiveness of the foam for its intended use in the implosion process. However, the foam strand must be scanned under a microscope to locate the individual spheres and cutting of the foam strand at the required distances from the spheres. Thus, in view of the very small size of the spheres, 40-50 .mu.m in diameter, for example, many of the spheres can be missed during scanning, with the resultant recovery rate of usable targets increasing the time and expense of the targets due to a large extent to the selecting of properly configured spheres or shells and filling of same with the fusion fuel, such as DT, prior to the foam encapsulation process which additionally increases the time and expense involved.